Add Vivawallet payment to Hikashop

-- url of the page with the problem -- : kosmima-gkiotli.gr
-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.5.1
-- PHP version -- : 5.4.45
-- Browser(s) name and version -- : Firefox 46.0.1

Hi,
I want to add a option for payment with vivawallet to hikashop and i don't know how. On the vivawallet's site they say that the only that I have to do is to add on my site the follow code:

<script src=" www.vivapayments.com/web/checkout/js "> </script>
<form action=/charge" method="POST">
<button
type="button"
class="simple"
data-vp-publickey="aBon7Oqqme4i/d6kPz3ICnIm/KEFXnavmpAGMR+dtK0="
data-vp-lang="el"
data-vp-description="Friendly description for you customer"
data-vp-amount="2000">
</button>
</form>

But I don't know where I have to add the code. Could you help me?

Thanks in Advance

Ola !

here is : www.webit.bz/hikashop-viva-payments-with-instalments-p-211.html